I think I've experienced what you're describing in most of my CB550's. This is where you try to advance to the next gear but can't. Wailing on the shifter doesn't do much good unless maybe you want to down shift. Which, of course you don't 'cause it's acceleratin' time. This reduces your faith in the machine temporarily, and makes you say "what the f@*K?"
Is it a sign of a dire future? I don't think so. For example, my 74, which I've riden since 75, still operates fine. The symptom hasn't gotten any worse. It's got 35-40K miles on it now.
One way to help prevent it's occurrance is to avoid short stroking the shift lever. Most of the time, moving the lever three quarters of the way of it's full stroke will get you the next gear. I find gear hangups to occur when I've had a bout of lazy foot.
Clearing the condition once it does occur, is speeded with abrupt drivetrain power changes. With the clutch engaged, quickly come way back on the power and just as quickly put it back on. You'll feel a slight bump on your shift foot, and find that all the gears are available again, like 3rd, which is what you wanted anyway. Then thiungs are fine until you have another bout of lazy foot.
Cheers,
